High accuracy photoplotting systems are well known in the art. These systems generate precision phototools which are used for semi-conductor masks, television arrays and for printed circuit boards (PCB) to write printed circuit board artwork onto a film emulsion. A typical photoplotter system, such as is marketed by the Gerber Scientific Instrument Company, consists of a magnetic tape drive, hard disk, computer, interactive graphics terminal, image processor, optical table having a moveable write platen for positioning the substrate, and a precision optical exposure device. The photoplotter system also includes a two-dimensional media carriage, a two-dimensional feedback interferometer, and electronics as is necessary to convert computer aided design (CAD) data to PCB artwork.
The optical scan and exposure system includes an optical exposure head (OEH). The optical table has a controller that employs a feedback position monitor, such as a laser interferometer, which continuously tracks the write platen position relative to the exposure head. The accuracy of the feedback position monitor is usually significantly greater than the platen positioning accuracy or that of the exposure head. For example, a Gerber 1434 direct imaging system has an accuracy of approximately 30 to 40 micro-inches and a Gerber 3235 direct imaging system has an accuracy of approximately 200 micro-inches. In contrast, a feedback position monitor often used with those systems (e.g., a Hewlett Packard interferometer) can have an accuracy of less than 1 micro-inch. Therefore, the performance of the table positioning mechanisms are limited by the mechanical component and servo system inaccuracies. These inaccuracies ultimately limit the overall performance of the photoplotter system.
It would be advantageous to have a method and apparatus for enhancing the accuracy of photoplotter systems by optically compensating for mechanical accuracy limitations. The present invention is drawn toward such a method and apparatus.